This invention relates to extruding apparatus and more particularly to apparatus for extruding plastic netting or the like.
More particularly, this invention is concerned with apparatus for extruding a plurality of filaments of plastic in such manner as to form a tube of netting in which the filaments overlap or intersect one another and are joined at intersections or "knots". Such extruding apparatus is shown, for example, in U.S. Pat. Nos. 2,919,467, 3,607,084, 3,560,306, 3,620,883 and in East German Pat. No. 52,801. Generally, tubular plastic netting is conventionally produced by forcing plastic melt through a circular series of extrusion orifices in an inner die and through an adjacent circular series of extrusion orifices in an outer die surrounding the inner die, the inner and outer dies being counterrotatable. After extrusion, the still partially molten plastic filaments forming the tube of netting are pulled down over a mandrel so as to maintain the cross-section of the tube at a desired diameter, and are quenched in a water bath. Upon cooling, the netting is often-times heated and stretched to reorient the molecular structure of the plastic so as to enhance strength, size and heat-shrink characteristics for subsequent packaging usage.
With prior extrusion apparatus, difficulty has been encountered in providing a uniform feed of plastic to the orifices of the inner and outer extrusion dies, nonuniform feed causing nonuniform extrusion of the netting. For example, the filaments extruded via some of the orifices may vary from those extruded via others. Also, it has been difficult to produce netting of consistently uniform mesh size or having uniform knot strength. It is particularly important that the netting have uniform knot strength because weak knots may separate in subsequent heating and stretching operations.
Problems have also been experienced in quenching the tubular netting in the water bath. More particularly, the downward movement of the netting into the water causes the water level on the inside of the tubing to be "pumped" below the water level at the exterior of the tubing. This unbalanced water level causes uneven cooling of the interior and the exterior of the netting and thus induces thermal stress gradients in the netting. It may also result in the netting sticking to the mandrel.
Other problems have been experienced in effectively sealing the bearings in the extruder housing which journal the outer die. Heretofore, plastic melt, in some instances, has worked its way into the bearings thus causing down time and lost production for the extruder.